The inflammatory response serves the purpose of eliminating harmful agents from the body. There is a wide range of pathogenic insults that can initiate an inflammatory response including infection, allergens, autoimmune stimuli, immune response to transplanted tissue, noxious chemicals, and toxins, ischemia/reperfusion, hypoxia, mechanical and thermal trauma. Inflammation normally is a very localized action which serves in expulsion, attenuation by dilution, and isolation of the damaging agent and injured tissue. The body's response becomes an agent of disease when it results in inappropriate injury to host tissues in the process of eliminating the targeted agent, or responding to a traumatic insult.
As examples, inflammation is a component of pathogenesis in several vascular diseases or injuries. Examples include: ischemia/reperfusion injury (N. G. Frangogiannis et al., in Myocardial Ischemia: Mechanisms, Reperfusion, Protection, M. Karmazyn, ed., Birkhuser Verlag (1996) at 236-284; H. S. Sharma et al., Med. of Inflamm., 6, 175 (1987)), atherosclerosis (R. Ross, Nature, 362, 801 (1993)), inflammatory aortic aneurysms (N. Girardi et al., Ann. Thor. Surg., 64, 251 (1997); D. I. Walker et al., Brit. J. Surg., 59, 609 (1972); R. L. Pennell et al., J. Vasc. Surg., 2, 859 (1985)), and restenosis following balloon angioplasty (see, R. Ross cited above). The cells involved with inflammation include leukocytes (i.e., the immune system cells—neutrophils, eosinophils, lymphocytes, monocytes, basophils, macrophages, dendritic cells, and mast cells), the vascular endothelium, vascular smooth muscle cells, fibroblasts, and myocytes.
The release of inflammatory cytokines such as tumor necrosis factor-alpha (TNFα) by leukocytes is a means by which the immune system combats pathogenic invasions, including infections. TNFα stimulates the expression and activation of adherence factors on leukocytes and endothelial cells, primes neutrophils for an enhanced inflammatory response to secondary stimuli and enhances adherent neutrophil oxidative activity. See, Sharma et al., cited above. In addition, macrophages/dendritic cells act as accessory cells processing antigen for presentation to lymphocytes. The lymphocytes, in turn, become stimulated to act as pro-inflammatory cytotoxic cells.
Generally, cytokines stimulate neutrophils to enhance oxidative (e.g., superoxide and secondary products) and nonoxidative (e.g., myeloperoxidase and other enzymes) inflammatory activity. Inappropriate and over-release of cytokines can produce counterproductive exaggerated pathogenic effects through the release of tissue-damaging oxidative and nonoxidative products (K. G. Tracey et al., J. Exp. Med., 167, 1211 (1988); and D. N. Männel et al., Rev. Infect. Dis., 9 (suppl. 5), S602-S606 (1987)). For example, TNFα can induce neutrophils to adhere to the blood vessel wall and then to migrate through the vessel to the site of injury and release their oxidative and non-oxidative inflammatory products.
Although monocytes collect slowly at inflammatory foci, given favorable conditions, the monocytes develop into long-term resident accessory cells and macrophages. Upon stimulation with an inflammation trigger, monocytes/macrophages also produce and secrete an array of cytokines (including TNFα), complement, lipids, reactive oxygen species, proteases and growth factors that remodel tissue and regulate surrounding tissue functions.
For example, inflammatory cytokines have been shown to be pathogenic in: arthritis (C. A. Dinarello, Semin. Immunol., 4, 133 (1992)); ischemia (A. Seekamp et al., Agents-Actions-Supp., 41, 137 (1993)); septic shock (D. N. Männel et al., Rev. Infect. Dis., 9 (suppl. 5), S602-S606 (1987)); asthma (N. M. Cembrzynska et al., Am. Rev. Respir. Dis., 147, 291 (1993)); organ transplant rejection (D. K. Imagawa et al., Transplantation, 51, 57 (1991); multiple sclerosis (H. P. Hartung, Ann. Neurol., 33, 591 (1993)); AIDS (T. Matsuyama et al., AIDS, 5, 1405 (1991)); and in alkali-burned eyes (F. Miyamoto et al., Opthalmic Res., 30, 168 (1997)). In addition, superoxide formation in leukocytes has been implicated in promoting replication of the human immunodeficiency virus (HIV) (S. Legrand-Poels et al., AIDS Res. Hum. Retroviruses, 6, 1389 (1990)).
One disease that can be treated via transplantation is diabetes mellitus. The incidence of diabetes mellitus is predicted to increase significantly in the next decade, and it already affects an estimated 130 million people worldwide. Diabetes affects 16 million Americans and consumes one out of every eight health care dollars. Despite the efficacy of insulin therapy, the devastating secondary complications, including nephropathy, neuropathy, retinopathy, and cardiovascular disease, can shorten life expectancy by as much as one third.
One method for treatment of diabetes is β-cell replacement therapy. This treatment is the best way to achieve ideal blood glucose control and stop the progression of the secondary complications of Diabetes. Islet transplantation is an attractive alternative to either insulin injection or whole organ pancreas transplantation. This method avoids the technical complications of solid-organ pancreas transplantation, related to thrombosis of the blood supply to the whole-organ allograft and the danger of activation of the digestive enzymes associated with the exocrine function. In addition, islet transplantation provides the opportunity to manipulate the islets prior to transplantation in order to decrease immunogenicity of the allograft.
Unfortunately, many recipients need 2 to 3 transplantations to achieve insulin independence even after transplantation of more than 250,000 Ieq. According to statistics, while the number of patients in the USA who are diagnosed with type I diabetes annually is about 30,000, the number of pancreas donated for transplantation is approximately 6000. These donated pancreases are not solely used for islet transplantation.
The majority of islet grafts are lost early after transplantation (within the first 3 days post-transplantation, more than half of islet grafts will die). Growing evidence implicates a nonspecific inflammatory reaction in the host microenvironment at the site of islet implantation and transplantation as one of the main reasons for islet graft early death.
Islets are believed to be highly sensitive to the toxic effects of inflammatory mediators. P-selectin exposed on activated platelets can also stimulate monocytes and macrophages to secrete chemokines that are deleterious to islets. The high concentrations of TNF-a, IL-1b, and NO generated at the site of the allograft may also have direct toxic effects on islets. Moreover, isolated human islets are also believed to express many genes involved in the generation of inflammatory responses after isolation. The expression of IL-1β, IL-8, MIP-2, MCP-1, and MIF have been found to rise after the isolation procedure, and after transplantation this upregulation may induce an intense inflammation and enhance subsequent specific immune response. As a result of this initial strong inflammatory response, subsequent antigen presentation would probably be promoted, leading to accelerated and reinforced cell mediated immunity in a later phase.
The detrimental effects of this instant inflammatory reaction may provide a reasonable explanation for the relatively low success rates in clinical islet transplantation and may explain the need for islets from several donors to obtain normoglycemia in the transplant recipient. Strategies to efficiently inhibit these cascade reactions at the time of transplantation and during the first postoperative days may be of great importance in improving the outcome of clinical islet transplantation.
It is well known that adenosine and some analogs of adenosine that nonselectively activate adenosine receptor subtypes decrease neutrophil production of inflammatory oxidative products (B. N. Cronstein et al., Ann. N.Y. Acad. Sci., 451, 291 (1985); P. A. Roberts et al., Biochem. J., 227, 669 (1985); D. J. Schrier et al., J. Immunol., 137, 3284 (1986); B. N. Cronstein et al., Clinical Immunol. and Immunopath., 42, 76 (1987); M. A. Iannone et al., in Topics and Perspective in Adenosine Research, E. Gerlach et al., eds., Springer-Verlag, Berlin, p. 286 (1987); S. T. McGarrity et al., J. Leukocyte Biol., 44, 411421 (1988); J. De La Harpe et al., J. Immunol., 143, 596 (1989); S. T. McGarrity et al., J. Immunol., 142, 1986 (1989); and C. P. Nielson et al., Br. J. Pharmacol., 97, 882 (1989)). For example, adenosine has been shown to inhibit superoxide release from neutrophils stimulated by chemoattractants such as the synthetic mimic of bacterial peptides, f-met-leu-phe (fMLP), and the complement component C5a (B. N. Cronstein et al., J. Immunol., 135, 1366 (1985)). Adenosine can decrease the greatly enhanced oxidative burst of PMN (neutrophil) first primed with TNF-α and then stimulated by a second stimulus such as f-met-leu-phe (G. W. Sullivan et al., Clin. Res., 41, 172A (1993)). Additionally, it has been reported that adenosine can decrease the rate of HIV replication in a T-cell line (S. Sipka et al., Acta. Biochim. Biopys. Hung., 23, 75 (1988)). However, there is no evidence that in vivo adenosine has anti-inflammatory activity (G. S. Firestein et al., Clin. Res., 41, 170A (1993); and B. N. Cronstein et al., Clin. Res., 41, 244A (1993)).
It has been suggested that there is more than one subtype of adenosine receptor on neutrophils that can have opposite effects on superoxide release (B. N. Cronstein et al., J. Clin. Invest., 85, 1150 (1990)). The existence of A2A receptor on neutrophils was originally demonstrated by Van Calker et al. (D. Van Calker et al., Eur. J. Pharmacology, 206, 285 (1991)).
There has been progressive development of compounds that are more and more potent and/or selective as agonists of A2A adenosine receptors (AR) based on radioligand binding assays and physiological responses. Initially, compounds with little or no selectivity for A2A receptors were developed, such as adenosine itself or 5′-carboxamides of adenosine, such as 5′-N-ethylcarboxamidoadenosine (NECA) (B. N. Cronstein et al., J. Immunol., 135, 1366 (1985)). Later, it was shown that addition of 2-alkylamino substituents increased potency and selectivity, e.g., CV1808 and CGS21680 (M. F. Jarvis et al., J. Pharmacol. Exp. Ther., 251, 888 (1989)). 2-Alkoxy-substituted adenosine derivatives such as WRC-0090 are even more potent and selective as agonists at the coronary artery A2A receptor (M. Ueeda et al., J. Med. Chem., 34, 1334 (1991)). The 2-alklylhydrazino adenosine derivatives, e.g., SHA 211 (also called WRC-0474) have also been evaluated as agonists at the coronary artery A2A receptor (K. Niiya et al., J. Med. Chem., 35, 4557 (1992)).
There is one report of the combination of relatively nonspecific adenosine analogs, R-phenylisopropyladenosine (R-PIA) and 2-chloroadenosine (Cl-Ado) with a phosphodiesterase (PDE) inhibitor resulting in a lowering of neutrophil oxidative activity (M. A. Iannone et al., Topics and Perspectives in Adenosine Research, E. Garlach et al., eds., Springer-Verlag, Berlin, pp. 286-298 (1987)). However, R-PIA and Cl-Ado analogs are actually more potent activators of A1 adenosine receptors than of A2A adenosine receptors and, thus, are likely to cause side effects due to activation of A1 receptors on cardiac muscle and other tissues causing effects such as “heart block.”
There remains a need for compounds that are useful for treating an inflammatory response caused by an immune response to transplanted tissue and that can enhance the survival rate of transplanted organs after transplantation.